SCRUBBING OF GASES CONTAINING TiCl{11

ABSTRACT

Industrial gas streams containing sizable amounts of HCl have in the past been efficiently scrubbed by large amounts of water or water solutions. But when a gas stream has also contained TiCl4 vapor, as when TiO2 has been produced via the &#39;&#39;&#39;&#39;chloride&#39;&#39;&#39;&#39; process, even scrubbing has not prevented an opaque plume from being vented to the atmosphere. The plume disappears or is markedly reduced in intensity, however, if a controlled amount of water is evaporated into the gas stream before scrubbing.

United States Patent Low [ 1 Sept. 12, 1972 [54] SCRUBBING OF GASESCONTAINING TICL4 [72] Inventor: David N. Low, Wilmington, Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

22 Filed: Sept. 30, 1971 21 Appl.No.: 185,254

[52] US, Cl. ..55/71, 23/87 T, 23/202 R [51] Int. Cl. ..B01d 47/00 [58]Field of Search....23/87 T, 202 R, 202 V; 55/71, 55/84, 220, 223

[56] References Cited UNITED STATES PATENTS 2,306,184 12/ 1942 Pechukas..23/87 X 3,118,732 1/1964 Richards et a1 ..23/202 V 3,219,411 11/1965Cheever et a1. ..23/202 TAIL 3,370,401 2/ 1968 Lucas et a1. ..55/903,607,049 9/ 1971 Weaver, Jr. et a1 ..23/202 V 3,615,163 10/1971Brozozowski ..23/87 T X Primary Examiner-Reuben Friedman AssistantExaminer-R. W. Burks Attorney-Donald A. Hoes [57] ABSTRACT Industrialgas streams containing sizable amounts of HCl have in the past .beenefficiently scrubbed by large amounts of water or water solutions. Butwhen a gas stream has also contained TiCl, vapor, as when TiO has beenproduced via the chloride process, even scrubbing has not prevented anopaque plume from being vented to the atmosphere. The plume disappearsor is markedly reduced in intensity, however, if a controlled amount ofwater is evaporated into the gas stream before scrubbing.

3 Claims, 2 Drawing Figures SIACK/ GAS GAS 4 SPRAY TOWER PETE-"TED 8E?12 I972 SHEET 1 OF 2 1N VENTOR DAVID N. LOW

BY MJM ATTORNEY PATENTEDSEP 12 m2 sum 2 or 2 INVENTOR DAVID N. LOW

KQMIJ O f N4 ATTORNEY SQRUBBING OF GASES CONTAINING TICL BACKGROUND OFTHE INVENTION In the manufacture of TiO by the chloride process, thereresults an anhydrous tail gas which contains a small amount of TiCl, andmuch larger quantities of HCl gas, CO, and inert gases including CO andN The gas stream is generally at or near atmospheric pressure and may bechilled.

The gas stream described above is a common one in the chemical industryexcept for the small amount of TiCl In the common case it is usualpractice to remove the HCl component in a scrubbing device, egventuri'scrubber, orifice scrubber or the like, where the HCl isabsorbed into a contacting water stream. Such practice is usually highlysuccessful because of the extremely low vapor pressure of HCl gas overHCl solution near atmospheric conditions. In a typical case in which theprocess exit gas is passed through a water spray tower before finallyventing to the atmosphere, it is practical to obtain a 200:1 reductionin the I-ICl concentration before the gas is released to the atmosphere.In such a case, the HCl is absorbed as a gas directly into the scrubberwater and there is usually no visible plume at the final stackdischarge. However, in similar waste gas streams which contain a smallfraction of TiCl vapor (for example 0.1 percent) and a larger fractionof HCl (for example percent) a fine mist, apparently of acid dropletsand titanium compounds, is formed when the gas stream first contactswater. These droplets in the mist are extremely small, being in themicron range, and as such are difficult to remove from the gas. If thedroplets are not removed, they present a highly visible stack plume eventhough the amount of acid and TiCl, appear to be negligibly small. Sincemost air pollution regulations limit the opacity of stack emission, itis highly desirable to eliminate or reduce the visible plume even thoughit may carry only a negligible acid loading.

To illustrate the foregoing, it is noted that in one installation aspray tower was found to remove only 67 percent of the acid componentfrom a gas stream initially containing 0.078% TiCl, and 4.5% HCl.However, in a test of this same spray tower with a synthetic gas streamcontaining the same amount of I-ICl but no C1,, the acid reductionthrough the spray tower was 99.5 percent. As further evidence of thedifficulty of scrubbing the visible plume from a gas stream with a TiCl,fraction, the following statement is taken from a translation publishedRussian literature: The Synthesis of Hydrolysis Products of TitaniumTetrachloride, by L. N. Shchegrov et al., Tr. VSES, Nauchn. Issled.Inst, Khim. Reaktivov, No. 25:470 8 (l 9 6 3) 7' 7 It was determined inpreliminary experiments that it is impossible to recover the unreactedtitanium tetrachloride completely by using only the abovedescribedsystem of liquid absorbents: the fog formed thereby, which constitutesthe products of partial hydrolysis of the titanium tetrachloride,suspended in the air stream, was not completely recovered even afterpassing through consecutively connected bubbling absorbers.

While it has thus been evident that a gas stream carrying a largeconcentration of HCl and a less amount of TiCl vapor could be cleared ofthe acid component with considerable ease if the effect of the TiClcomponent were somehow removed, an economical and efficient means forachieving this has not previously been available.

SUMMARY OF THE INVENTION In accordance with the invention an improvementis provided in a method wherein an anhydrous gas stream composedpredominantly of inert gases and containing appreciable quantities ofHCl and TiCl, is to be contacted with an aqueous base liquid in anamount in excess of that required to saturate the stream with water tothereby absorb the RC1 in the form of an HCl solution. More particularlyit has been found that an improvement in such a method can be achievedleading to elimination or at least substantial reduction in the opacityof the gas upon being vented to the atmosphere. The improvement involvesevaporating'an initial and restricted quantity of water into the streamof anhydrous gas at least one-twentieth sec. prior to the regularscrubbing operation in which the gas is contacted with a saturationamount of water. The initial quantity of water evaporated into thestream must be less than that required to saturate the stream at theexisting temperature and pressure conditions but greater than thatstoichiometrically required to react with the TiCl, in the stream toform TiO The finding of the present invention thus is that with a gasstream containing TiCl, and HG], a critical controlled water treatment,i.e. to produce a gas containing water in subsaturation amounts, inadvance of the usual water scrubbing step, remarkably and surprisinglyreduces the opacity of the stream upon being vented to the atmosphere.The exact reasons for this phenomenon are not fully known but either oftwo possibilities, or a combination of both, appear to be involved. Thuson the one hand, it is considered that by initially limiting the amountof water that is available for reaction with the TiCl (the reaction isknown to be virtually instantaneous) only the more soluble forms oftitanium oxychlorides are produced, i.e. in preference to TiO Ti(OH) orless soluble oxychlorides, and these are then readily removed in thesubsequent scrubbing step. Alternatively, or perhaps cojointly, it ispossible that the limited quantity of water tends to produce, at leasttemporarily, a highly acidic condition which itself quickly promotesdissolution of the TiCl component, either as such or as some other formof a titanium compound. In either event, the TiCl, component of the gasstream is unable to serve in the generation of mist upon being contactedwith water in subsequent scrubbing step.

In any case, the quantity of water used to pretreat the gas stream, i.e.treat it in advance of the typical water scrubbing step, must be withinspecific limits. The minimum quantity needed is that which istheoretically required to chemically combine with the TiCl in the gasstream according to the equation TiCl 2H 0 TiO 4HCl This is not toimply, however, that TiO is necessarily the reaction product, sinceother hydrolysis products are also possible depending upon temperatureand other conditions.

As regards the maximum amount of water which can be used in thepretreatment, it has been demonstrated that a successful reduction inthe intensity of the plume is not achieved if the stream becomessaturated with water such that fine particle size, i.e., submicron size,liquid water droplets are formed. Such liquid droplets would, onceformed, be carried through the scrubbing operation and be eventuallydischarged into the atmosphere giving the appearance of a dense whiteplume. Advantageously, the amount of water to be supplied and sprayedinto the stream of gas will be regulated at a level which is below 100percent, i.e. l to 99 percent of that which would saturate the gas atthe temperature and pressure conditions existing upon the wateraddition. In general, it is preferred that, to be on the safe side,there be used an amount which is less than 50 percent of the water thatwould saturate the gas.

If for example one is given a tail gas from the chloride TiO processcontaining 0.3 volume TiCl flowing at a rate of 4,000 s.c.f./m. at 70C.and 3 p.s.i.g. then it can be calculated that, as a minimum, 0.144gal/min. H will have to be supplied in the pretreatment for that is theamount stoichiometrically required to react with the TiCl, in the tailgas. Ignoring, temporarily, the cooling effect that occurs uponintroducing water, it can also be calculated from known vapor pressuredata that 7.6 gal/min. H 0 would be required, assuming 100 percentmixing efficiency, to saturate the particular stream with water. Thus aflow rate could be selected between the extremes of 0.144 gal/min. and7.6 gal/min. Should it be found that a cooling occurs to, say, 60C., thelatter or maximum would have to be corrected to 4.36 gal/min.

It is to be understood that the determinative factor is the quantity ofwater actually evaporated into the gas stream during the course of thepretreatment. There is no disadvantage in introducing an excess of waterprovided that other factors such as mixing efficiency, contact time andthe like are such as to prevent the entire amount of supplied water fromactually evaporating. Most conveniently, however, there will beintroduced to the gas stream a predetermined quantity of water which iswithin the range described above and under conditions, i.e. withsufficient atomization evenly across the cross section of the stream soas to promote uniform contact. In this regard it will be understood thatwhile the hydrolysis of TiCl, occurs virtually instantaneously, at leasta small equilibration period is needed prior to contacting the streamwith gross quantitles of scrubbing water. In general at leastone-twentieth of a second and preferably at least one-half of a secondis required for equilibration to take place. By extending the length ofthe conduit between the location of the pretreatment inlet and thescrubbing apparatus, the equilibration period is increased.

As a practical matter, a suitable subsaturation condition is bestascertained by observing the intensity of the plume that results as thescrubbed gas is vented to the atmosphere. Between the extremes of toomuch water and too little water, each of which will tend to give anintense, highly opaque plume, there will be an intermediate range wherethe amount of water of water can be readily controlled so as to achievea notable reduction in plume opacity.

While the invention will be described with reference to the use ofliquid water as the pretreating liquid, it is to be understood thatsatisfactory results can be achieved if the water contains additiveswhich are essentially of an inert nature. For example, acids such asl-ICl, which may be recycled from the process itself, may be provided inthe water to be used. Direct evaporation of liquid water appears to beessential as steam does not afford a comparable reduction in thevisibility of the plume unless caused to condense for subsequentevaporation.

The invention is applicable, as indicated hereinbefore, to anhydrous gasstreams containing a major proportion, by volume, of inert gas andlesser proportions of HCl and TiCh. It is not uncommon that tail gasesfrom a chloride TiO process will contain on the order of 2 to 20% HCland 0.01 to 0.5% TiCl these percentages being on a volume basis. The lowpercentage of the latter is readily attainable by condensationprocedures which, perform subsequent to the oxidation process, aretypically used to recover unreacted TiCl In any case the process of thepresent invention is readily applicable to gases of the foregoingcompositions. Other constituents in the gas appear to have little effectupon the results which are obtained although it may be desirable toemploy a special treatment to separate certain noxious gas componentsbefore venting to the atmosphere. For example, the elimination of C1 gascan be facilitated by a caustic scrub.

It is to be understood that the means employed to actually separate theI-ICl from the gas stream, i.e., the scrubbing apparatus, is not acritical feature of the invention. Thus the process of the invention hasbeen demonstrated in conjunction with a variety of scrubbing devicesincluding venturi scrubbers, orifice scrubbers, demisting devices, spraytowers, etc. The choice of the scrubbing device or devices depends uponthe exact nature of the gas to be treated, the efficiency of the HClremoval which is desired, and economics.

FIGS. 1 and 2 illustrate, in schematic form, embodiments of theinvention for first pretreating a gas stream with a small amount ofwater followed by a scrubbing operation which is performed in the usualway. The details of these figures will be described in connection withExamples I and II, which follows:

EXAMPLE I The gas treating apparatus corresponds generally to thatdepicted schematically in FIG. 1 wherein the tail gas stream at arelatively high velocity is sequentially subjected to (a) a waterpretreatment under conditions of a short holdup time, (b) a scrubbingstep using a large quantity of water in a conventional orifice scrubber,(c) a gas-liquid separation, (d) a second scrubbing step using a largequantity of water, (e) a second gas-liquid separation and (f) passage ofthe gas to the vent stack.

The gaseous stream is that resulting from a chloride TiO process andtypically is analyzed as follows on a volume percent basis:

2 CO 87.l% 2 HCl l2.8% TiCl, 0.1% H 0 0.0%

The gas stream enters duct 11, a 20 inch diameter corrosion resistantconduit, on the average at a rate of 3,320 s.c.f./min., a pressure of 2p.s.i.g., and a temperature of 74C. A fine spray of water is directedfrom the nozzle 13 substantially across the entire cross section of duct1 1 to pretreat the gas with a limit amount of liquid water, i.e. undersubsaturation conditions. The flow water to nozzle 13 is controlled byflowmeter 14. An annular plate 15, located 4 feet from nozzle 13 andhaving a 12 inch diameter opening, serves to avoid upstream migration ofwater.

The gas stream then passes to a conventional gas scrubbing system.Initially it is treated in an orifice scrubber formed by an annularplate 16, having an opening about 6.75 inches in diameter, above whichis positioned a pair of scrubbing water inlets 17. The inlets arecircumferentially spaced about 11 to direct, tangentially with respectto the duct interior, streams of water into contact with the gas streamthereby to absorb the HCl component. The distance from nozzle 13 toinlets 17, forming a zone shown generally as 18, is approximately 3.5feet. A thermometer 19 measures the temperature of the subsaturated gasstream in zone 18.

The mixture of gas and HCl solution so produced then passes to a cyclonewhich serves as a gas-liquid separator with the HCl solution, containingabout 70 weight percent of the HCl in the gas, being drawn off forconcentration, recycling, or other use. The gas fraction is then given asecond scrubbing in a 20 foot high, 7 foot diameter spray tower to whichwater is being fed at a rate of 2,800 gal/min. This removes the bulk ofthe remaining 30 weight HCl. After a second cyclone separation, the gasis vented to the atmosphere through a stack.

In operation the quantity of water passing through nozzle 13 isregulated to give the least amount of stack a plume, which is a rate ofabout 1 gal/min. This provides a cooling effect which is indicated bythe temperature dropping from 74 to 59C. in zone 18. The quantity ofwater introduced through inlets 17 at the orifice scrubber is regulatedat the rate of 80 gal/min. Under these conditions, little or no visiblesmoke plume is observed at the vent stack as the scrubbed gas passesinto the atmosphere.

By calculation a gas stream of 3,320 s.c.f./min. at 59C., 2 p.s.i.g.,would require 2.95 gal/min. of water to saturate the stream. The lgaL/min. rate thus being charged to the nozzle 13 produces only aboutone-third of theoretical saturation.

Also by calculation it is determined that with 0.1 percent by volumeTiCl, in the gas stream, 0.035 gaL/min. of water is required totheoretically react with the TiCl to produce Ti0 Thus the l gal/min.rate is well in excess of the minimum that would be needed.

Using the same arrangement the amount of water being introduced atnozzles 13 is varied over a wide range with, in each case, little orplume being observed at the vent stack. Thus at a 0.35 gal/min. rate andeven at a 2.5 gal/min. rate, little or no plume is observed. However, ifthe amount of water introduced at nozzles 13 exceeds 2.95 gall/min. suchthat a saturated condition is produced in zone 18, a heavy white plumebegins to form. A similar condition is observed if no water isintroduced at 13 or if it is used in an insufficient quanti- EXAMPLE nThe gas treating apparatus corresponds generally to that schematicallydepicted in FIG. 2 wherein a rela- V tively low velocity stream issequentially subjected to (a) a water pretreatment under conditions oflong holdup time, (b) a scrubbing step using a large quantity of anaqueous solution in a conventional venturi scrubber, (c) a gas-liquidseparation, (d) a demister and (e) passage of the gas to the vent stack.

The gaseous stream is essentially the same as that described in Example1 except that the volume HCl is about 5 percent and the volume TiCl, isabout 0.1 1 percent.

The gas stream enters spray tower 31, a 5 foot diameter 20 foot highcorrosion resistant vessel, at an average rate of 1,435 s.c.f./min., apressure of 6 p.s.i.g., and a temperature of l0C. A thin stream of wateris directed from nozzle 33 into vessel 31 to pretreat the gas with alimited amount of liquid water, i.e. under subsaturation conditions.Although the stream of water is not evenly distributed across the crosssection of the vessel, the large residence time of the gas thereincoupled with the strong affinity of TiCl, for water provides thenecessary contact. The flow of water to nozzle 33 is controlled byflowmeter 34. Unvaporized water, in the form of an acid solution, isdrawn off into sump 35 through liquid seal 36.

The gas stream found to now be at about+l5C. then passes trough an 8inch diameter corrosion resistant duct 37, in which temperature ismeasured by a thermometer 38, to a low 3inches H 0) pressure dropventuri scrubber formed by the discharge of about 30 gal/min. recycledHCl solution (averaging 20-31 weight percent concentration) from anozzle 41 downwardly into a throat constriction. Some 60-70 weightpercent of the HCl component of the gas is removed by the combinedaction of the spray tower and venturi.

The mixture of gas and HCl solution then passes to a cyclone whichseparates the gas and the HCl solution. The gas fraction then passes toa demister of type described in Lucas et al US. Pat. No. 3,370,401, inwhich an inert wire mesh forms the fibrous bed. The bulk of the HClremaining in the gas is thus removed.

In operation the quantity of water passing through nozzle 33 isregulated at an average rate of l gal/min. but about 98 percent byweight of this amount is removed at 36. With the volume of gas beingtreated, a subsaturated condition exists in duct 37. Under theseconditions little or'no visible smoke plume is observed at the ventstack as the scrubbed gas passes into the atmosphere. A distinct plumebegins to form, however, if the water passing to power 31 is stoppedaltogether or if it is increased to the point where the submicron sizedroplets are being formed by a saturated condition therein.

lclaim:

1. In a method wherein an anhydrous gas stream composed predominantly ofinert gases and containing appreciable quantities of HCl and TiCl, iscontacted with an aqueous based liquid in an amount in excess of thatrequired to saturate the stream with water to thereby absorb the HCl inthe form of an HCl solution, the improvement wherein at leastone-twentieth second prior to said contact an initial quantity of wateris evaporated in the stream, said quantity being less than that requiredto saturate the stream at the existing temperature and pressureconditions but greater than that stoichiometrically required to reactwith the TiCl in the stream to form TiO 2. Method according to claim 1wherein said initial

2. Method according to claim 1 wherein said initial quantity of water isless than 50 percent of that required to saturate the stream.
 3. Methodaccording to claim 1 wherein the gas stream is a tail gas from achloride TiO2 process.